This invention relates to a hydrotreating catalyst and a method for its preparation. More particularly, the invention relates to a hydrotreating catalyst prepared from gamma alumina precalcined at a temperature above 1375.degree. F.
In the refining of liquid hydrocarbons derived from mineral oils and other sources, it is often necessary to subject the liquid hydrocarbon or fraction thereof to hydrotreating. Hydrotreating is a process for reducing the concentration of nitrogen and sulfur components in a hydrocarbon feedstock so that, when the product hydrocarbon is eventually combusted, less SO.sub.2 and NO will form, and hence present less of a pollution problem. In addition, it is often desirable to remove nitrogen from such feedstocks in order to protect hydrocracking catalysts and the like which deactivate in the presence of nitrogen.
In general, hydrotreating is accomplished by contacting the feedstock containing nitrogen and/or sulfur with a catalyst in the presence of hydrogen and under conditions, including elevated temperature and pressure, such that the sulfur components are converted to H.sub.2 S and the nitrogen components to NH.sub.3, both of which are separated from the desulfurized and denitrogenated liquid product.
A typical hydrotreating catalyst comprises particles containing a Group VIII active metal component and a Group VI active metal component supported on a refractory oxide such as alumina. Oftentimes, phosphorus components are also present in the catalyst to improve its activity by increasing its acidity. One catalyst which has been successfully employed on a commercial basis consists essentially of molybdenum, nickel, and phosphorus components supported on gamma alumina. A typical preparation procedure for such a catalyst is as follows: particles of hydrated alumina are firstly formed into a desired size and shape by extruding the hydrated alumina through a die having circular or clover leaf-shaped openings therein and cutting the extruded matter into particles (or extrudates) of 1/16-1/2 inch lengths. After calcining at a temperature of about 1150.degree.-1250.degree. F., the resulting gamma alumina extrudates are in a condition to be contacted with an impregnating solution comprising dissolved salts of molybdenum and nickel in phosphoric acid. The impregnated extrudates (or composites) are then subjected to a final calcination at a temperature around 900.degree. F. to convert the impregnated metals to their oxide forms.
Quite unexpectedly, it has now been found that, if the above procedure is only slightly altered, the resulting catalyst is substantially more active for hydrotreating purposes. More specifically, it has been found that, if the first calcination, usually termed the precalcination, is conducted at a temperature of at least 1375.degree. F., and preferably at 1450.degree.-1600.degree. F., then the final catalyst will have increased activity for removing sulfur and nitrogen components from liquid hydrocarbons under typical hydrotreating conditions.
In one embodiment, therefore, the invention comprises a hydrotreating catalyst composition comprising molybdenum, nickel, and phosphorus components supported on gamma alumina prepared by a method including the step of impregnating a support comprising gamma alumina precalcined at a temperature of at least 1375.degree. F. with components of nickel, phosphorus, and molybdenum. In an alternative embodiment, the invention comprises a method for preparing a hydrotreating catalyst wherein particles of gamma alumina precalcined at at least 1375.degree. F. are impregnated with a solution comprising phosphoric acid containing dissolved nickel and molybdenum components, and the resulting composite is calcined at an elevated temperature. In yet another embodiment, the invention comprises an improved hydrotreating process wherein the improvement resides in the use of a catalyst prepared by calcining a composite comprising molybdenum, nickel, and phosphorus components impregnated into gamma alumina precalcined at at least 1375.degree. F. Usual precalcination temperatures herein are above 1400.degree. F.